1. Field of the Invention
The present invention relates to a gas-shielded tandem welding method and particularly to a two-electrode welding method that has high deposition efficiency per heat input, has good mechanical properties of a weld metal, and can decrease the amount of spatters by significantly reducing arc interference between electrodes, which has been an unavoidable problem.
2. Description of the Related Art
Gas-shielded arc welding is the most general-purpose and widespread welding technique at present, but higher efficiency has been required. Gas-shielded arc welding is broadly divided into consumable electrode welding methods such as a metal active gas (MAG) welding method and a metal inert gas (MIG) welding method and non-consumable electrode welding methods such as a tungsten inert gas (TIG) welding method. Among these welding methods, MAG and MIG welding methods are overwhelmingly excellent in terms of efficiency, and the efficiency has been improved in both the methods. Herein, to reduce the number of passes by increasing the deposition amount, which is the most effective way for achieving high efficiency, a larger amount of welding wire needs to be fed and melted in a short time.
When the feeding speed of a single-electrode wire is increased in MAG and MIG welding methods, the wire is excessively heated and is thus melted before reaching an arc. Consequently, hanging droplets become unstable and a large amount of spatters is generated. Furthermore, when the number of revolutions of a feeding roller is increased, the feeding speed itself becomes unstable, which affects an arc. Moreover, an increase in electric current excessively increases an arc force and a molten pool is dug deeply, resulting in a flow defect. This poses problems such as cause of undercut and humping defects and degradation of bead spread. Therefore, there is a limitation to increase the feeding speed.
Thus, welding methods described below have been conventionally performed.
[1-1: Tandem Arc Welding]
A tandem arc welding method in which the total amount of wire melted is increased by generating arcs using two electrodes has been proposed and is now widespread (e.g., refer to Japanese Unexamined Patent Application Publication Nos. 2004-1033, 2003-053545, and 2006-247695 and Japanese Patent No. 4089755). However, such a welding method in which two electrodes are close to each other poses a large problem of arc interference. A magnetic field in a rotational direction is generated around a conducting wire through which an electric current passes. When two electrodes come close to each other, the mutual interference produces an attractive force in the case where the polarities of electric currents are the same or a repulsive force in the case where the polarities are opposite. Therefore, the directions of the arcs are also affected and the directivity thereof becomes unstable. Consequently, droplets moving from a wire to a molten pool fly out due to the effect and a large amount of spatters is generated. To address such a problem, some measures have been taken, such as use of a pulsed current, use of an optimum phase, and shortening of an arc length by voltage adjustment.
However, such measures are not fundamental solutions and the problem of generation of spatters has not been solved yet. If the distance between electrodes is increased, arc interference is reduced and thus the amount of spatters is decreased. However, this poses problems in that a curved material to be welded cannot be tracked, it makes difficult to enter a narrow portion because of the upsizing of a welding apparatus, and a lack-of-welding area is increased at the beginning and end of welding. In terms of ease of operation, the distance between electrodes is desirably small because a welding apparatus with a small size can be provided. Furthermore, if an electric current of one of the two electrodes is decreased, the effect on the arc of the electrode having a high electric current is decreased and the amount of spatters is decreased. However, since the arc of the electrode having a low electric current has a weak arc force and is subjected to a strong arc force exerted by the electrode having a high electric current, the amount of spatters may be increased. Therefore, the total amount of spatters is not decreased. As described above, in the tandem arc welding method, the problem of spatters generated by arc interference has not been solved yet.
In view of an influence exerted on the performance of a weld metal, the tandem arc welding method is also not preferred. That is, since the total amount of heat provided to a base metal is large due to the generation of arcs from both electrode wires, the cooling rate of a weld metal portion is decreased. As a result, the size of crystal structures increases and thus the strength, toughness, and the like easily decrease.
[1-2: Hot Wire TIG Welding]
In a TIG welding method in which a tungsten electrode that generates an arc is separated from a filler wire, if an arc current of TIG is increased to increase the melting rate of the wire, the front end of the tungsten electrode is melted by heating and damaged. Therefore, the melting energy cannot be significantly increased. To make it easy to melt a filler serving as the wire, it has been disclosed that a wire or welding rod that is normally not energized is energized and the temperature of the wire or welding rod is increased by electric resistance heating to improve the melting property (e.g., refer to Japanese Patent Nos. 2610819 and 4151777). Herein, TIG welding has an advantage of generating almost no spatters. However, since the arc current of TIG is not so high, the TIG arc easily loses its directivity due to magnetic interference caused by energizing the filler. Consequently, the TIG arc becomes unstable and lack of penetration readily occurs. In addition, since the TIG welding method uses a non-consumable electrode, the deposition rate is fundamentally low. Even if the filler is easily melted, the efficiency of TIG welding cannot compete with the efficiency of MAG or MIG welding. Thus, a further increase in the efficiency of MAG or MIG welding has been demanded.
[1-3: Hot Wire MAG Welding]
A technology in which a technique of using a filler metal in TIG welding in the form of an energized filler is applied to MAG or MIG welding has been proposed (e.g., refer to Japanese Unexamined Patent Application Publication Nos. 2004-148369 and 2-169183 and Japanese Patent No. 3185071). Since a filler is melted in a molten pool, droplets are not formed in an arc and thus spatters are not generated regardless of the degree of energizing current. Thus, it is desired that the melting rate is increased by further increasing the temperature of a filler wire through application of high energizing current. However, as in the case of TIG welding, filler energizing current affects the arc of a leading electrode. Even though spatters are not generated from both electrodes unlike the tandem arc welding, the amount of spatters generated in an arc electrode is increased. To address such a problem, some measures have been taken such as use of a pulse for an arc electrode and an energized filler electrode, but such measures are not fundamental solutions. An increase in the heat input is suppressed compared with the tandem arc welding because the filler electrode does not generate an arc. However, if the filler energizing current is increased to achieve high efficiency, the total amount of heat is increased and the strength, toughness, and the like are easily decreased.
[1-4: Double Wire Welding]
In the hot wire MAG welding, one of the two electrodes is employed as an energized filler and an electric current is supplied to the arc electrode and filler electrode using two independent welding machines. Herein, a technology similar to the technology of hot wire MAG welding has been proposed (e.g., refer to Japanese Unexamined Patent Application Publication No. 3-275280 and International Publication No. WO02/018086). In this technology, instead of the supply of an electric current with two welding machines, part of an arc current of a leading electrode is shunted to an energized filler serving as a trailing electrode using a single welding machine. Thus, welding is performed with a simple apparatus. However, also in this method, an arc of the leading electrode is subjected to interference and spatters are generated. In addition, since the electric current shunted to the filler serving as the trailing electrode is not a fixed value but a value proportional to the electric current of the leading electrode, the trailing electrode is directly affected by the fluctuation of the electric current of the leading electrode. Therefore, the electric current is not kept constant and becomes unstable, which easily causes melting defects of the filler. In principle, the polarities of electric currents of the arc electrode serving as the leading electrode and the filler electrode serving as the trailing electrode are opposite. For example, when the wire has a “positive” polarity and the base metal has a “negative” polarity in the leading electrode, the wire has a “negative” polarity and the base metal has a “positive” polarity in the trailing electrode.
When the polarities of electric currents are opposite as described above, an interference force with which the arcs of the electrodes repel each other is exerted and the arc of the leading electrode faces forward in a welding direction. This shifts a molten pool immediately below the arc forward and the digging force is increasingly reduced. Consequently, incomplete penetration easily occurs. In addition, since spatters do not enter the molten pool, but scatter to an unwelded portion in front, the amount of spatters attached to a work is increased. As in the hot wire MAG welding, an increase in the heat input is suppressed compared with the tandem arc welding because the filler electrode does not generate an arc. However, if the filler energizing current is increased to achieve high efficiency, the total amount of heat is increased and the strength, toughness, and the like are easily decreased.
[1-5: Type of Welding Wire]
In the case of the above-described welding method in which a single molten pool is formed with two electrodes, the same welding wire is generally used for the two electrodes in terms of ease of operation. However, in the case of a two-electrode method in which the leading electrode is an arc electrode and the trailing electrode is a filler electrode, the function required for the leading electrode and the function required for the trailing electrode are different from each other. Consequently, if the same welding wire is used, some problems may be caused. For example, when both an arc electrode wire and a filler electrode wire are solid wires, the wire of the filler electrode is not easily melted and an unmelted wire is easily generated. Furthermore, since the filler electrode does not generate an arc, an oxidation reaction caused by contact between droplets (wire-melted liquid) and atmosphere gas generated in a high-temperature arc space substantially does not occur (refer to FIGS. 5A and 5B). Thus, when a wire having a composition that is optimized for an arc electrode by containing a reducing element with a strong oxygen affinity, such as Ti, is employed as a wire of the filler electrode, the process “a reducing element is oxidized and then discharged in the form of slag” is not undergone and thus an excessive number of inclusions (Ti particles) are left in a weld metal and the toughness significantly degrades.
To increase the strength and toughness of the weld metal, in general, elements such as Mo and B that improve hardenability need to be added from a wire. However, if these elements are added to a solid wire, the wire drawability is decreased. Therefore, annealing and pickling need to be repeatedly performed in the production process, which increases the cost. Such elements can be added without affecting the wire drawability if a flux-cored wire is employed. However, when such a flux-cored wire is used for the arc electrode, the depth of penetration is disadvantageously decreased compared with the case of a solid wire.